A thermal flow meter that measure a flow rate of gas is configured to include an air flow sensing portion for measuring a flow rate, such that a flow rate of the gas is measured by performing heat transfer between the air flow sensing portion and the gas as a measurement target. The flow rate measured by the thermal flow meter is widely used as an important control parameter for various devices. The thermal flow meter is characterized in that a flow rate of gas such as a mass flow rate can be measured with relatively high accuracy, compared to other types of flow meters.
However, it is desirable to further improve the measurement accuracy of the gas flow rate. For example, in a vehicle where an internal combustion engine is mounted, demands for fuel saving or exhaust gas purification are high. In order to satisfy such demands, it is desirable to measure the intake air amount which is a main parameter of the internal combustion engine with high accuracy. The thermal flow meter that measures the intake air amount guided to the internal combustion engine has a bypass passage that takes a part of the intake air amount and an air flow sensing portion arranged in the bypass passage. The air flow sensing portion measures a state of the measurement target gas flowing through the bypass passage by performing heat transfer with the measurement target gas and outputs an electric signal representing the intake air amount guided to the internal combustion engine. This technique is discussed, for example, in JP 2011-252796 A (PTL 1).
However, it is known that pollutants such as an exhaust gas discharged from its internal combustion engine or other vehicles may be mixed into the inside of the intake pipe of the internal combustion engine, and the pollutants may be adhered to an air flow sensing portion arranged in the bypass passage, so that a heat transfer surface of the air flow sensing portion may be polluted. Similarly, it is known that a splash such as a water droplet generated from a vehicle running ahead during a raining or snowing day may be mixed into the intake, and such a water droplet may be scattered to the air flow sensing portion and may be adhered to the heat transfer surface of the air flow sensing portion.
For example, if the heat transfer surface of the air flow sensing portion is polluted by pollutants, a thermal conductivity of the heat transfer surface changes, so that it is difficult to obtain a discharge characteristic of an initial (shipping) state. In this case, even when a measurement target gas of the same flow rate makes contact with the heat transfer surface, an output value is different from that of the initial state and has an error. In addition, if a water droplet is adhered to the heat transfer surface of the air flow sensing portion, an output waveform has a spike shape due to its vaporization heat, so that it is difficult to obtain an accurate output until the water droplet is vaporized or removed from the heat transfer surface.
For such problems, for example, JP 2009-109368 (PTL 2) discusses a technique of avoiding particle pollutants such as minute carbon or liquid pollutants such as oil or water droplets that are not easily filtered through centrifugal separation from reaching the heat transfer surface of the air flow sensing portion.
In the device discussed in PTL 2, there is proposed a bypass passage curved at an angle of 90° or larger in an upstream side from a plate-shaped sensor element. In this technique, the bypass passage is curved at an angle of 90° or larger on a virtual plane perpendicular to a sensor formation surface of the plate-shaped sensor element and parallel to a flow direction, and a gap is provided between a sensor formation surface side of the plate-shaped sensor element, a backside thereof, and a wall surface of the bypass passage.
Once a liquid pollutant such as oil or a water droplet described above is adhered to the inner wall surface of the bypass passage, it moves inside the bypass passage slowly not to generate a centrifugal force. That is, when pollutants such as a water droplet scatted inside the intake pipe are input to the inside of the bypass passage, most of the pollutants are adhered to the wall surface of the bypass passage before it reaches the air flow sensing portion. Once a pollutant is adhered to the wall surface, its movement speed is sufficiently slower than that of the air flow inside the bypass passage. Therefore, the pollutants are guided to an inner circumference side of the curved bypass passage having a fast flow speed.
In the device discussed in PTL 2, the bypass passage is curved at an angle of 90° or larger on a virtual plane perpendicular to the sensor formation surface of the plate-shaped sensor element and parallel to a flow direction, and a gap is provided between a sensor formation surface side of the plate-shaped sensor element, a backside thereof, and the wall surface of the bypass passage. Therefore, it is possible to avoid particle or liquid pollutants guided to the inner circumference side of the bypass passage from reaching the sensor element portion.